Conventionally, many methods for producing a porous film, which comprises drawing a film containing a polyolefin resin and an inorganic bulking agent in a uniaxial or biaxial direction to form pores (voids) connected to each other in the film, have been proposed. Such porous films are used for a variety of applications such as sanitary materials, medical materials, building materials and battery separators.
In recent years, demands for disposable diapers for adults are increasing, and requirements of high gas permeability and moisture permeability have been increasing year by year for back sheets which cover moisture (water) absorptive materials. With respect to diapers for adults, a high pressure is locally applied in the inside thereof, as compared with diapers for children, whereby urine may pass through a gas permeable back sheet and leak out of the diaper, such being problematic. Further, there is an increasing tendency to use gas permeable films for back sheets also in an application of disposable napkins. In the application to napkins, as compared with diapers, a higher pressure is applied, and the surface tension of blood is low as compared with urine, whereby blood is likely to leak through pores as compared with urine.
Further, in recent years, gas permeable films have been used as back sheets for napkins. In such a case, blood absorbed in a moisture (water) absorptive material tends to be seen through the back sheet, and accordingly it has been desired to increase hiding properties of back sheets. Further, when a disposable diaper or napkin is fixed to an underwear by means of an adhesive tape, components contained in the adhesive tape may move to the porous film and make the opaque porous film transparent, whereby the inside may be seen through the film, and such a problem is more severe in a case of napkins.
Accordingly, a porous film which has excellent anti-leakage properties and anti-bleeding properties, which has excellent hiding properties and which has excellent masking properties at an adhesion portion is required. Further, a porous film which does not leak e.g. urine or blood and passes only moisture has been required, in a case where the porous film is used as materials for disposable diapers and sanitary products.
As a means to overcome the above-mentioned problems, a method of blending a bulking agent with a base resin as the main components, and adding at least one member selected from the group consisting of a fatty acid amide, a liquid paraffin and a sorbitan fatty acid ester as an accessory component to obtain a film has been proposed (JP-A-62-250038). According to experiments conducted by the present inventors, uniform drawability without surface irregularity at the time of film production and flexibility of the film to be obtained can be achieved by addition of such an accessory component, but no well balanced film can be obtained in view of anti-bleeding properties against a hydrophilic liquid, gas permeability, moisture permeability and masking properties at an adhesion portion.
Further, JP-A-58-15538 discloses an example wherein liquid polybutadiene, liquid polybutene or terminal hydroxy liquid polybutadiene is added as a hydrocarbon polymer (including one having side chains) as an accessory component. JP-A-58-149925 discloses an example wherein liquid polyisoprene is added. However, according to experiments conducted by the present inventors, it was found that anti-bleeding properties against a hydrophilic liquid are low, drawing tends to be non-uniform, and the hiding properties are low, even if such an accessory component is added.
Further, Japanese Patent No. 1763293 discloses addition of a silicone oil and/or a polyglycerol fatty acid ester surfactant as an accessory component and that addition of such a component increases flexibility, tensile strength, tear strength, uniform drawability, hydraulic pressure resistance and the like of a film. However, according to experiments conducted by the present inventors, it was found that anti-bleeding properties and hiding properties are inadequate with respect to the polyglycerol fatty acid ester. In a case of the silicone oil, good anti-bleeding properties are obtained as compared with the above fatty acid ester or a hydrocarbon polymer, however, it was found that the anti-bleeding properties significantly decrease and the anti-leakage properties are also inadequate, even if the resin composition including the accessory component is the same, unless the orientation state of the film and the pore structure such as pore size and porosity are controlled to be within specific narrow ranges. For example, in a case of a film drawn 3.0 times×3.0 times in a biaxial direction, the orientation state of the film tends to be isotropic, whereby not only permeability of a liquid in a thickness direction tends to be too high, but also the anti-bleeding properties are inadequate since the pore size is too large.